1. Field of the Invention
The invention relates to the utilization of multiple-station in-place foaming of reactive polymeric resin components introduced into articles which are being advanced along a production line. Although use of the invention is not so limited, it is more particularly adapted to methods for integrating an insulating layer or blanket of cellular resin foam in appliance housings such as refrigerator cabinets.
2. Description of the Prior Art
The "foaming" of refrigerator cabinets is a well-established and defined industry of long standing. The operation involves incorporating an insulating blanket of resin foam by forming this in-place in the walls of the cabinets as they progress along a production line in a factory. Historically, the major production lines have employed a series of fixtures or jigs on which the cabinets are supported on movable transports to advance the cabinets through successive processing operations. Along with the moving fixtures, it has been conventional to employ low-pressure (50-100 psi), agitator-equipped, mixing heads for uniting the reactive polymers, and then open pouring the foam mix into the walls of each cabinet, while it is being supported by its moving fixture.
The moving fixture concept necessarily involves complex mechanisms for physically transporting the fixtures through the various processing stations in order to subject the cabinets to the several operations required. Starting, stopping, opening, closing, reorienting, and similar handling operations, which must be accomplished in the processing of the cabinets, cause complexity of the equipment utilizing this moving fixture approach. Furthermore, such equipment is not readily adapted to meet, from an economic standpoint, changing production conditions. Likewise it is apt to be limited in terms of production output. Additionally, the low pressure, agitator-equipped mixing heads typically used require solvent or air flushing between pour operations and can not satisfactorily handle some of the more highly reactive polymeric compositions, nor insure rapid dispersion of the foam mix throughout the whole area of the cabinet or other article.
The alternate use of stationary fixtures for supporting the articles to be foamed, in combination with conveyor arrangements for moving the articles to such fixtures and positioning or orienting an article in each fixture, is also not new. Such arrangement has the advantage of simplifying the transport mechanism used in moving the articles to and from the fixtures, and in the handling of electrical and mechanical connections, since the connections do not move with the articles in this case. Moreover, in this system it is much simpler to increase production output of a product line, since it is only necessary to parallel additional fixture stations to which the articles can be delivered. This contrasts with physically increasing the length of path of the moving fixture arrangement, in order to insert additional fixtures in the train of the fixtures as would be required in that arrangement. Likewise, if a stationary fixture is made inoperative for purposes of clean-up or maintenance, this does not interrupt the processing of articles at other stations, which is not true of the other system.
More recent innovations have incorporated the stationary fixture concept with high pressure, impingement mixing (reaction injection mixing) foam dispensing heads. The use of high pressure (1,000-3,000 psi) metering and mixing allows more reactive chemical formulations to be used, and this reduces curing time and consequently speeds up production. It also insures better distribution of foam mix throughout the article to be "foamed".
With the use of multiple-station foaming systems, problems arise because of the necessity of feeding the reactive liquid resin components to each of the foaming stations which of necessity must be physically separated some distance. It is economically disadvantageous to use individual component storage, pumping, temperature conditioning, metering and recycling units for each of the stations, since such systems are relatively complex and expensive. On the other hand the provision of a single source of component feed, etc. for all stations of a multiple-station installation requires long conduit lines. This leads in turn to problems of adequately maintaining temperature and compositional uniformity throughout the system. It also increases the possibility of line swelling due to pressure surges, resulting in lead or lag of component feed and thus producing temporary off-ratio foam mixes. It is accordingly a principal object of this invention to effect improvement in respect to these problems.